The present invention is related to a combined arc melting and tilt casting apparatus used, e.g. for the manufacture of bulk metallic glass materials.
Tilt casting is reported to produce the best fatigue endurance in Zr-based bulk metallic glasses. Incorporating the alloying and casting facilities in a single piece of equipment reduces the amount of laboratory space and capital investment needed. Eliminating the sample transfer step from the production process also saves time and reduces sample contamination. The glass forming ability in many alloy systems, such as Zr-based glass-forming alloys, deteriorates rapidly with increasing oxygen content of the specimen.
Bulk metallic glasses are amorphous metals, with a diameter larger than 1 mm, that solidified without detectable crystallization. Upon heating from the solid state these alloys exhibit a glass transition, after which they remain metastable for a finite length of time in the super-cooled liquid region, before crystallizing. Enhanced stability against crystallization is usually achieved by alloying multiple elements with significant difference (>12%) in atomic radius and negative heats of mixing among constituent elements. The critical casting diameters of known BMG alloys typically range from 1 mm to 100 mm. BMG alloys have been found in many different alloy groups (Pd-, Mg-, Ln-, Zr-, Ti-, Fe-, Co-, Ni- and Cu-based systems) and new alloys have been discovered and reported with a variety of different properties. By casting BMG alloys, without cold working or heat treatment, complex shapes can be produced with excellent mechanical properties: purely plastic deformation up to a yield strain of typically 2%, resulting in tensile strength from 1500 MPa to 5500 MPa, with Youngs modulus from 70 MPa to 275 MPa. The lack of grain boundaries in the BMG materials also results in very accurate surface finish and enhances corrosion resistance. Several recent reviews testify to the widespread interest in these materials both from a fundamental science perspective and for practical applications.
Different methods may be used to produce amorphous metals, each with its own advantages and disadvantages, whose relative importance depends on the alloy composition and the intended purpose. Strictly speaking, an amorphous solid is called a glass only if it was formed when a liquid state underwent a glass transition. Thus, metallic glasses are formed by melting the constituents to obtain a molten alloy with the desired composition, and then quenching the molten alloy below its glass transition temperature. Often, pre-alloying to obtain the desired composition and quenching to the glassy state are entirely separate processes, carried out in different apparatuses. Prior to the discovery of bulk glass-forming alloy compositions, the rapid quenching methods required to avoid crystallization for most metallic glass-formers meant that these materials could be produced in glassy form only as thin ribbons, foils or wires. The significance of BMGs can be attributed in large part to the versatility of metal mold casting methods in producing different shapes, as well as larger objects, out of metallic glass. If needed, casting can be followed by additional shaping or patterning steps—involving machining operations or superplastic forming in the viscous super-cooled liquid region—but usually the pre-alloying and casting steps are decisive for the quality of the final part.
For alloying, induction melting and arc melting under inert atmosphere arc commonly used, both with water-cooled copper crucibles. Both methods allow precise control of the melting process in laboratory scale production. Typically, the process chamber is repeatedly evacuated to a pressure below 1×10−3 Pa and backfilled with purified argon, then purged of any remaining oxygen by titanium gettering before the constituent metals are melted for alloying. It is standard practice to flip over the pre-alloyed ingot and remelt it several times to ensure that its composition is uniform. When the process chamber must be opened to air to flip the ingot, renewing the inert atmosphere takes time, wastes argon, and risks contaminating the BMG with oxygen. Oxygen is harmful for BMG manufacture because, for some of the phases whose crystallization competes with glass formation, the crystallization kinetics are enhanced by oxygen. As a result, BMG samples contaminated with oxygen typically are inferior to high-purity samples. So not only is it quicker and more economical to perform the necessary manipulation of the ingot without repeatedly opening the process chamber: it also produces better samples.
For casting BMG, variants of metal mold casting are most commonly used. The method of quenching described in the earliest reports of bulk metallic glass formation in the Pd—Ni—P system—and earlier work on marginally bulk glass forming Pd—Si based alloys—did not involve metal mold casting. For some alloys, direct quenching of remelted pre-alloyed ingots in a fused silica container, especially in combination with fluxing, is still the preferred method for making high-quality BMG samples. However, it is difficult to produce complex shapes by this method, and the dimensional tolerances and surface quality obtained by direct quenching methods are not as good as those obtained by metal mold casting. A relatively simple version of metal mold casting consists of induction melting an pre-alloyed ingot in a fused silica crucible that has an orifice at the bottom, and then applying gas pressure to eject the molten BMG forming alloy into a mold placed beneath the crucible. High vacuum induction melting and argon pressure casting apparatus, with a linear feedthrough for moving the fused quartz crucible from the induction coil to the mold orifice, was found to be very versatile in easily producing different specimen shapes, such as bars, rods, wedges, rings, bar, and “dogbone” tensile specimen. In a laboratory setting—where process conditions are often varied—it is particularly convenient to be able to view the sample through the quartz crucible during melting. However, because the same quartz crucible is a possible source of oxygen contamination, it may sometimes be preferable to use other crucible materials, such as graphite. More sophisticated casting methods such as suction casting, tilt casting, squeeze casting and cap casting may produce better quality specimens, e.g., because they can more consistently and more uniformly fill the mold and achieve higher cooling rates. In particular, BMG specimens produced by the combination of tilt casting with cap casting or squeeze casting, compared to those produced by conventional tilt casting, have been reported to exhibit larger critical casting diameter and improved ductility.